1. Field of the Invention
The present invention relates to optical beam transmission systems. More specifically, the present invention relates to electro-optical light valves used in such systems.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In a number of conventional optical systems aberrations in the phase of a transmitted beam may occur during propagation through a distorting medium. One method employed in compensating for these phase aberrations has been to pass a phase conjugated replica of the perturbed optical beam back through the distorting medium. A phase conjugated replica of an optical beam has a complex spatial amplitude equivalent to the complex conjugate of the amplitude of the original beam. Following traversal of the distorting medium the phase conjugated replica of the perturbed optical beam is transformed into a relatively undeformed version of the original optical beam.
One conventional optical phase conjugation technique utilizes a combination of a wavefront sensor and a phase conjugating crystal to imprint the spatial phase modulation obtained from the wavefront sensor onto the crystal in a closed loop operation. Low sensitivity to weak signals has limited the efficacy of this technique in certain applications. In particular, phase conjugating crystals typically require incident signals on the order of watts to tens of watts per square centimeter to provide adequate phase conjugation. This requirement may preclude inclusion of such phase conjugating crystals in, for example, certain high sensitivity imaging systems.
Recently liquid crystal light valves (LCLVs) have also been utilized in phase conjugation of optical signals. (see Garibyan, et al, "Optical Phase Conjugation by Microwatt Power of Reference Waves via Liquid Crystal Light Valve", Optics Communications, Vol. 38, no. 1, July 1981 and also E. Marom and U. Efron, "Phase Conjugation of Low-Power Optical Beams Using Liquid-Crystal Light Valves", Optics Letters, Vol. 12, 504, July 1987.) The sensitivity of these LCLVs to low intensity signals has been demonstrated by Garibyan to be orders of magnitude greater than the corresponding sensitivity typically exhibited by phase conjugating crystals.
Within the optical light valve system described by Garibyan, a "photosensible semiconductor" layer and a liquid crystal layer are separated by a dielectric mirror. A voltage is applied externally between the photosensitive semiconductor and liquid crystal layers. Hereinafter, the photosensitive semiconductor layer will be referred to as a "photoconductor". A reference beam and a signal beam are incident on the dielectric mirror and respective portions of each are transmitted to the photoconductor. The remaining portions of each beam are then reflected back through the liquid crystal layer. An interference pattern is formed near the photoconductor by interaction of the transmitted portions of the reference and signal beams. As is well known, this interference pattern subsequently induces spatial variations in the impedance of the photoconductor. Moreover, as the liquid crystal and photoconductor are effectively electrically connected in series, these impedance changes are mirrored as spatial variations in the voltage drop across the liquid crystal. In this manner the interference pattern is impressed upon the liquid crystal layer.
The liquid crystal layer modulates the phase of the propagating optical energy in response to an applied voltage. As a result, the portion of the reference beam reflected back through the liquid crystal layer will be modulated in response to spatial variations in the voltage drop induced by the interference pattern. A specific geometrical orientation of the reference beam relative to the longitudinal axis of the liquid crystal layer provides a portion of the reflected reference beam with a phase conjugated version of the original signal beam.
Although the system disclosed by Gariban is sensitive to signals of low intensity, the magnitude of the phase conjugated signal is limited by the saturation level of the photoconductor. Specifically, the proportion of the reference beam penetrating the dielectric mirror and illuminating the photoconductor is fixed once a particular dielectric mirror has been chosen. It follows that the magnitude of the reference beam may not be increased above a certain level without saturating the photoconductor and thereby overwhelming the interference pattern.
As the magnitude of the phase conjugated signal is proportional to the magnitude of the reflected portion of the reference beam, constraints on the intensity of the reference beam effectively determine the maximum intensity of the conjugated signal. Further, although increasing the reflectivity of the dielectric mirror increases the allowed reference beam intensity, the sensitivity of the system to weak signal beams will be correspondingly lowered.
The relatively weak intensity of the phase conjugated beam in the system proposed by Garibyan limits the utility of this system for a variety of applications unless a separate amplifying element is used. Inclusion of an amplifying element, however, often complicates overall system design since a reimaging of the conjugated signal will then be typically required.
Hence a need in the art exists for an optical light valve system for providing an amplified phase conjugated replica of a potentially low intensity optical beam.